The present invention relates to motor starters. More particularly, the invention relates to a motor starter apparatus and method utilizing a current by-pass and/or current isolator arrangement.
It is known that when certain electric motors are started they can draw up to ten times their rated current for a limited period. Thus, motor starters are utilized to control the electric current supplied to the motor. A common method of controlling current utilized by motor controls is the use of thyristors or SCR's. However, a drawback to using SCR's is the large amount of heat generated. For example, typically a 1.5 volt drop occurs across each pair of thyristors of a motor starter and for a three-phase 600 ampere starter, this generates about 2.5 kilowatts of undesirable heat, which requires substantial cooling. This problem has been alleviated in the prior art by employing a switching mechanism that includes a by-pass arrangement connected in a parallel with the motor starter. After the motor has been started, the by-pass allows current to by-pass the thyristors and thus, lessen the heat dissipated by the motor starter.
An isolator arrangement is another switching mechanism often utilized in motor starters. An isolator arrangement is connected in series between the motor and the current source and is commonly required in order to permit isolation of the motor from the current source. Typical by-pass arrangements and isolator arrangements incorporate the use of fully rated, high performance contactors that are both very bulky and expensive. Also such arrangements are typically separate stand alone mechanisms that need substantial control power and have to be hard-wired or bussed into the associated circuits. Accordingly, it is an object of this invention to provide means whereby the above problems associated with the starting of motors are at least alleviated.
U.S. Pat. No. 4,953,890 contemplates a bypass and isolator arrangement utilizing solenoids that are directly mounted on the motor starter assembly rather than a separate stand alone mechanism. The solenoids are used to move and hold movable contacts which engage the stationary contact surfaces for the bypass and isolator arrangements. However, utilizing solenoids to tnove and hold contactors for bypass and isolator purposes is undesirable for a number of reasons.
For example, the typical bypass and isolator switching times required for motor starters is on the order of seconds while solenoids operate in a fraction of a second, an unnecessary speed which is obtained at the sacrifice of the high cost of solenoids. Thus, the solenoid is an unnecessarily expensive approach and it is desirable to utilize a more cost effective bypass and isolator approach. Furthermore, the energy needed to drive a solenoid contactor arrangement closed is large. Solenoids, therefore, require the use of an undesirably large transformer. This further increases the size, costs, and power consumption of the motor starter. Solenoids also create an unde, sirable high impact on the contact surfaces. Accordingly, it is an object of this invention to provide means whereby the above problems associated with the use of solenoids are at least alleviated.